Friday, December 8, 2017

Stopping a neutrino beam; measuring their interaction cross-section

Neutrinos are popularly known as the particles that go through anything and everything.  Neutrinos from beta decay can escape from the best shielded nuclear reactor, and neutrinos from nuclear fusion escape from the center of the sun.  Neutrinos interact only via the weak interaction, which is indeed weak.  But, that doesn't mean that they can go through anything - the IceCube Neutrino Observatory recently demonstrated experimentally that it is possible to stop a beam of neutrinos, in a paper published in Nature (also freely available on the arXiv).

To do this, IceCube used two tricks. 

First, it use extremely energetic neutrinos, with energies above 1 TeV (1 tera-electron volt, or 1012 electron Volts), extending up to 1 PeV (1 peta-electron volt, or 1015 eV), millions of times more energetic than neutrinos from nuclear fusion or radioactive ion decay.  The cross-section (probability) for neutrinos to interact rises with energy (linearly at first, then moderated to scale roughly as Energy0.3.  So, at an energy of 30 TeV (the rough mid-point of the measurement) the cross-section is several million times higher than it is for neutrinos from radioactive decay.  Of course, there aren't that many neutrinos this energetic, but, at 1 cubic kilometer in volume,  IceCube is big enough to collect a good sample.  The analysis used 10, 784 energetic muons from neutrinos that passed through at least some of the Earth.

Second, it used a very thick absorber - the Earth.  With this, the measurement was quite simple.   It Compared to a baseline of near-horizontal neutrinos that traversed only a relatively small amount of matter, energetic near-vertical neutrinos were absorbed going through the Earth.  The figure above shows the predicted transmission probability (= 1 - absorption probability), as a function of neutrino energy and zenith angle; the latter shows how much Earth matter was traversed.   

There are of course many complications - experimental uncertainties on the neutrino energy, neutral current interactions, where a neutrino may emerge from the Earth with a lower energy than it entered, modelling the material within the Earth, etc., but the result clearly showed that neutrinos are absorbed at about the expected rate.  More precisely, the best-fit cross-section was. 1.3 +/- 0.5 times the predictions of the Standard model where I have combined the statistical and systematic uncertainty.  It was not trivial to find a good definition for the neutrino energy range for which this measurement applies, because different methods give somewhat different energy ranges, but we settled on a method that returned a range from 6.3 TeV to 980 TeV.  For comparison, the highest energy measurements at an accelerator laboratory only reached 0.37 TeV - our measurement reaches order of magnitude higher energies than than.  The figure below puts this in perspective, comparing our measurement with the previous accelerator work.  The cross-sections (y axis) are divided by the neutrino energy so that everything fits on the graph better; otherwise, it would span many orders of magnitude.

I have to mention that this was the dissertation work of my (now graduated) graduate student, Sandra Miarecki.  Sandy had a very interesting preparation for graduate school - she was a career US Air Force Pilot, serving many roles, including as a test pilot, before retiring from the Air Force and coming to graduate school in Berkeley.   After graduate school, she became an Assistant professor at the US Air Force Academy.   The LBNL news center has a very nice article about her.

The Nature article also recieved a fair amount of press coverage.  I will just mention one article,  in Symmetry magazine, which goes into more detail about the analysis than other press writeups.


  1. robots2005 AI32080March 14, 2018 at 10:01 AM

    I think neutrinos can be used in the late 2020s to image grains of Uranium on Earth . The idea is to place single radiowave or microwave detectors inside a captured comet. The comet has been brought L4 or L5. The rim around Earth's globe, looking away from the Sun, will emit neutrinos only from the crust and above. These neutrinos will interact with ice inside a comet and produce radiowaves and microwaves. These neutrinos will have originated from a cylinder around 150km deep and I'm hoping centimeters in diameter. After one rotation of Earth, there will be a second subsequent shower of microwaves and/or radiowaves and the location of the Uranium's elevation and surface speck will be triangulated as a slightly different comet interior shower will be produced. There are trillions of such detectors placed in the comet and they are transparent to the wavelength measured. Too long a wavelength and you have GPR ringing as the radiowaves reflect of the sublimated comet microcrystals 2 or 3 metres beneath the surface; the idea is for many detections to be directional so an arrow at Earth can be reconstructed.
    This has geopolitical implications but Uranium is a long term vulnerability.

  2. robots2005 AI32080March 15, 2018 at 8:13 AM

    I've figured out more details. Cherenkov radiation through ice creates a cone of microwaves just as it creates a cone of blue light. The cone is weaker and shaped a little differently. I'll use the technique of transmitting images through parallel conductive rods. Microwaves at the source create a near field. One microwave may not effectively be imaged by one Pb (or whatever material) rod but 4 or 40 microwaves should be imagible along parallel rods. Somehow the rods are connected to a near field detector at the far end of the comet. The strength of the near field signal gives the location of the impact as well as the Uranium source on Earth. This will work on Earth too aimed at Fukushima or wherever.

  3. robots2005 AI32080March 19, 2018 at 7:32 AM

    I have a method to locate nuclear materials. Microwaves form a Cherenkov light cone after a neutrino impacts ice. These microwaves will travel along a “wire medium lens”. A cylinder of ice faces Earth or a nuclear materials location. This is followed by an Aztec Pyramid lens with the wide end facing the ice. Microwaves will travel from the ice to the fat end of the pyramid. After the narrow end of the pyramid are microwave sensors. The microwaves will be focused enough to be detected. Two neutrinos from a nuclear source, impacting the ice at different locations, will form microwave cones that can be used to triangulate exactly where the uranium is. This will scale from comets to local detectors. It suggests the Navy should go battery with tidal energy recharging, and that nuclear materials should be stored at Australia and guarded until better heavy lift is available.

  4. robots2005 AI32080March 23, 2018 at 12:25 PM

    This is a directional neutrino detector procedure that should be good enough for spotting leaky wet storage containers. With better microwave sensors around the corner, it will eventually offer real time location data of nuclear materials, though ice cooling or changing is not good in many locales.:
    a wire medium super lens is dumped in water an a rectangular ice prism is made. At 100 metres, the microwave diffraction Cherenkov Cone of nuclear material in a long term storage tank is maybe 2-3 metres above a leak in the ground. That is about 3 degrees difference for average neutrino light cone overlap. Underground, the ice block with the wire medium superlens is tilted so the wire face is at 56-59 degrees and only the 3 degrees of single leak source illuminates a cumulative plastic sensor grid placed along the fair field side (facing tilted up)of the superlens. Microwave Cherenkov Cones are diffuse and maybe a ring, so if there is too much overlap this will fail. Mobile detectors are trickier with the mantle. I hope someone else will figure out how to make this work underground and I think geo-neutrinos have a "brighter" future than do others.